1. Technical Field
The present disclosure relates to integrated circuits and in particular the detection and measurement of a current in such a circuit. The present disclosure applies in particular to current detection circuits or current sense amplifiers present in non-volatile memories to read the state of memory cells. The present disclosure more generally applies to any circuit in which a current must be detected or measured.
2. Description of the Related Art
Various battery-powered devices comprise a non-volatile memory such as an EEPROM or Flash memory. Such memories are also present in contactless integrated circuit cards, which are powered from electric signals picked up by their antennas. It is therefore desirable that volatile memories may operate in a wide supply voltage range and in particular at low supply voltages, and that their electrical consumption may be as low as possible. These objectives of supply voltage and electrical consumption are hard to reach when memory reading and writing operations are subjected to heavy constraints in terms of execution speed.
The reading speed of a memory is significantly affected by the speed performances of the memory sense amplifiers, which tend to decrease with the supply voltage. Reading a memory cell of a non-volatile memory generally involves converting a current coming from the memory cell into a voltage and comparing the voltage obtained to a reference voltage. The conversion of cell current into voltage is performed by a current detection circuit. An example of such a circuit is shown in FIG. 1.
In FIG. 1, the circuit comprises a reference branch, a measurement branch and a comparator CP1. The reference branch comprises a P-channel MOS transistor referred to as P11, an N-channel MOS transistor referred to as N11, an inverter I1 and a current source CS1 supplying a reference current Irf1. Transistor P11 comprises a source terminal receiving a supply voltage Vdd of the circuit, and gate and drain terminals connected to the drain of transistor N11. Transistor N11 comprises a source terminal connected to the ground through current source CS1 and connected to a gate terminal of transistor N11 through inverter I1. The measurement branch comprises a P-channel MOS transistor referred to as P12, an N-channel MOS transistor referred to as N12, an inverter 12 and a current source CS2 symbolizing the current to be detected or measured. Transistor P12 comprises a source terminal receiving a supply voltage Vdd of the circuit, and a gate terminal connected to the gate of transistor P11. Transistor P12 comprises a drain terminal connected to the drain terminal of transistor N12. Transistor N12 comprises a source terminal connected to the ground through current source CS2 and connected to a gate terminal of transistor N12 through inverter 12. The comparator CP1 compares the voltage present on the drain terminals of transistors P12 and N12 with the voltage present on the drain terminals of transistors P11 and N11 (or on the gate terminals of transistors P11 and P12), and supplies an output voltage Vout representative of the comparison result. Transistors P11, P12 form a current mirror having a transmission rate equal to one, to transmit all the current Irf1 circulating in the reference branch to the measurement branch. Transistors in cascode configuration with an inverter and a source follower stage with a unitary feedback loop are used to obtain a short precharge duration of the circuit in which the current must be measured, independent of the circuit capacitive load. The circuit of FIG. 1 allows a rapid precharge to be obtained independently of the capacitive load of the circuit whose current is to be measured, up to a supply voltage of 1.6 V. Below this value, the circuit in which the current is to be measured is not insufficiently biased, and the reading speed deteriorates.
There is therefore a need for a current detection or measurement circuit keeping good performances in terms of detection speed and electrical consumption, up to supply voltages lower than 1 V. There is also a need for a circuit which is simple and having low electrical energy consumption.